Deposition of palladium



United States Patent 3,285,754 DEPOSITION OF PALLADIUM Norman Maynard Hopkin, London, England, assignor to Johnson, Matthey & Company Limited, London, England, a British company No Drawing. Filed June 14, 1963, Ser. No. 287,773 Claims priority, application Great Britain, July 5, 1962, 25,789/ 62 24 Claims. (Cl. 1061) This invention relates to improvements in and relating to the deposition of palladium from aqueous solutions and more particularly to the method of deposition commonly known .as immersion or electroless plating in which deposition is effected solely by immersing the article to be plated in the plating solution, and without the passage of an external electric curent.

Whilst electro-deposition is the method most generally employed for the deposition of one metal on another and is applicable to the majority of plating requirements, there are, nevertheless, certain circumstances in which electro-deposition becomes difficult, if not impossible, to carry out. For example, where it may be desired to plate a small base metal article, such for example, as an electrical contact, with a noble metal surface as a protection against tarnishing, considerable difficulty may be experienced in arranging such article as the cathode in an electroplating bath. Or, again, in the manufacture of printed circuits, the conducting surfaces of which it may be desired to protect, the nature of the article to be plated may be such as to preclude the use of electrodeposition procedures.

As a result of these difiiculties, a method of deposition, which does not involve the use of an electric current, but results by electrochemical replacement or reduction, has come into use and has been found satisfactorily to overcome the above difiiculties and enable satisfactory deposits to be applied in these circumstances.

This invention, as stated above, is concerned solely with the immersion method of plating and has for its object to provide a method of, and a plating bath for, immersion or electroless plating of palladium by means of which sound and bright deposits of palladium can be obtained on a variety of metal bases.

. The applicant has found that the deposition of palladium can be satisfactorily obtained on a metal base by the immersion or electroless plating method by the use of a plating bath comprising an aqueous solution of a complex nitrite-palladite compound of the general formula:

wherein A represents an alkali metal or a hydrogen cation; R represents a monovalent acid radicle;

R" represents a divalent acid radicle, and

a is 1, 2, or 3,

and provided said solution is buffered to a pH within the range of from 2 to 5.

In carrying out the invention, the plating bath is preferably buffered to the required pH by the addition to the bath of a potassium or sodium or other alkali metal salt of .a weak acid; or the corresponding acid may first be added and then potassium hydroxide, sodium hydroxide or other alkali hydroxide introduced until the required pH is obtained.

Examples of suitable weak :acid salts, which may be used in carrying out the invention, are the following, namely: sodium citnate, sodium acetate, sodium oxalate, sodium tartrate, sodium phosphate, sodium borate and sodium metaborate or the corresponding potassium salts.

The acid radicle may be the suphate, citrate, oxalate, tartrate, acetate, phosphate, selenate, perchlorate, nitrate, chloride, bromide or fluoride.

When, as will be found preferable in practice, an alkali metal is used toform the cation constituefit, the latter is preferably sodium or potassium.

Suitable palladium compounds which may be employed in carrying out the invention are:

The above compounds may be prepared in the following manner:

(1) This compound may be prepared by reacting potassium-pallado-nitrite with hydrochloric .acid, according to the following equation:

(2) This compound may be prepared by reacting potassium-pallado-nitrite with sulphuric acid, according to the equation:

(3) This compound may be preparedby boiling a solution of pall-ado-nitrous acid H Pd(NOg) prepared by passing a solution of potassiumapallado-nitrite through a cation exchange colunm with sulphuric acid. This reaction is exemplified by the following:

Compound (4) may 'be prepared in a similar manner to compound (1), but substituting nitric acid for hydrochloric acid. Similarly, compounds (5 )and (6')'may be prepared in the same manner as compound (3), but using selenic acid and perchloric acid respectively instead of sulphuric acid.

The invention also comprises a method for the immersion or electroless deposition of palladium on, a metal base, said method comprising immersing the base in a bath comprising an aqueous solution of -a complex nitritopalladite compound of the general formula:

Ag[Pd(NO2)aR4-n] or Al[ ,d(NO7)nR4 a] 2 wherein:

A represents an alkali metal or a hydrogen cation;v R represents a monovalent acid radicle;

R" represents a divalent acid nadicle and a is l, 2, or 3,

and said solution being buffered to a pH within the range of from 2 to 5.

In carrying out the invention in practice, the improved bath preferably contains 1-30 grams per litre of palladium and is advantageously operated at" a temperature of from 20 C. to 70 C. for periods varyirig'from 15 mins.

K Pd(NO 80 Examples (1) The bath contained 9.8 grams/litre of palladium and was buffered with 2.46 grams of sodium acetate to a pH of 3.28.

(2) The bath contained 7.7 grams/litre of palladium and was buffered with 0.82 gram of sodium acetate to a pH of 3.8.

(3) The bath contained 1 gram/litre of palladium and was buffered with 0.4 gram of sodium acetate to a pH of 4.18.

(4) The bath contained 10 grams/litre of palladium and was buffered with 4.2 grams of Rochelle salt to a pH of 3.77.

(5) The bath contained 25 grams/litre of palladium and was buffered with 8.46 grams of Rochelle salt to a pH of 3.36.

(6) The bath contained grams/litre of palladium and was buffered with 10 grams/litre of citric acid and 4.6 grams/litre of potassium hydroxide to a pH of 3.25.

(7) The bath contained 10 grams/litre of palladium and was buffered with grams/litre of sodium tetraborate (Na B O .10H O) to a pH of 2.0.

(8) The bath contained 10 grams/litre of palladium and was buffered with 20 grams/litre of sodium metaborate (NaBO to a pH of 3.40.

My improved plating bath, in accordance with this invention, may be used for the plating of base metals and also of noble metals. In the latter case, however, the noble metal, with the exception of silver, must be in contact with a base metal. Particularly satisfactory results may be obtained on copper, silver, brass, aluminum bronze, phosphor bronze, beryllium copper and mild steel. The bath will produce a smooth bright deposit of palladium and will be found particularly suitable for use in the manufacture of printed circuits for providing a corrosion-protective surface on the copper circuitry thereof or for protecting silver contacts from tarnishing during storage. It will be found that printed circuitry or contacts protected by means of the'invention can readily be soldered without cleaning after many months of storage.

What I claim is:

1. A plating bath for the immersion or electroless deposition of palladium on a metal base, said bath consisting essentially of an aqueous solution of a complex nitrito-palladite "compound having a general formula selected from the group:

A represents a cation selected from the group consisting of alkali metal and hydrogen cations; R represents a monovalent acid radicle; R". represents a divalent acid radicle, and a is a number selected from the group of numbers 1, 2 and 3, said solution containing from 1 to grams per liter of palladium, and said solution also containing a buffering agent comprising an alkali metal salt of a weak acid in sufficient amount to give said solution a pH within the range of from 2 to 5. i 2. A plating bath according to claim 1 in which said acid radical is selected from the group consisting of the sulphate, citrate, oxalate, tartrate, acetate, phosphate,

selenate, perchlorate, nitrate, chloride, bromide and fluoride.

3. A plating bath according to claim 1 in which A is an alkali metal cation derived from the group consisting of potassium and sodium cations.

4. A plating bath according to claim 1 in which said weak acid salt is selected from the group of salts consisting of sodium citrate, sodium acetate, sodium oxalate, sodium tartrate, sodium phosphate, sodium borate, sodium metaborate and the corresponding potassium salts.

5. A plating bath for the immersion or electroless deposition of palladium on a metal base, said bath consisting essentially of an aqueous solution of a complex nitrito-palladite compound having a general formula selected from the group:

wherein:

A represents a cation selected from the group consisting of alkali metal and hydrogen cations;

R represents a monovalent acid radicle;

R" represents a divalent acid radicle, and

a is a number selected from the group of numbers 1, 2

and 3, said solution containing from 1 to 30 grams per liter of palladium, and said solution having been buffered to a pH within the range of from 2 to 5 by the addition to said solution of a buffering agent comprising a weak acid and of an alkali metal hydroxide as required.

6. A plating bath for the immersion or electroless deposition of palladium on a metal base, said bath consisting essentially of an aqueous solution consisting essentially of potassium-dichloro-dinitrito-palladite,

K Pd (N0 Cl and remainder water, said solution containing from 1 to 30 g. per liter of palladium.

7. A plating bath for the immersion or electroless deposition of palladium on a metal 'base, said bath consisting essentially of an aqueous solution consisting essentially of sodium-dichloro-dinitrito-palladite Na Pd 2012 and remainder water, said solution containing from 1 to 30 g. per liter of palladium.

8. A plating bath for the immersion or electroless deposition of palladium on a metal base, said bath consisting essentially of an aqueous solution consisting essentially of potassium-sulphato-dinitrito-palladite,

and remainder water, said solution containing from 1 to 30 g. per liter of palladium.

9. A plating bath for the immersion or electroless deposition of palladium on a metal base, said bath consisting essentially of an aqueous solution consisting essentially of sodium-sulphato-dinitrito-palladite Na Pd H Pd (N0 50 and remainder water, said solution containing from 1 to 30 g. per liter of palladium.

11. A method for the immersion or electroless deposition of palladium on a metal base, said method comprising immersing the base in a plating bath maintained at a temperature of about 2070 C., said bath consisting essentially of an aqueous solution of a complex nitritopalladite compound having a general formula selected from the group wherein:

A represents a cation selected from the group consisting of alkali metal and hydrogen cations;

R represents a monovalent acid radicle;

R" represents a divalent acid radicle, and

a is a number selected from the group of numbers 1, 2 and 3,

and said solution containing about 1-30 grams/litre of palladium and also containing a buffering agent comprising an alkali metal salt of a weak acid in sufficient amount to give said solution a pH Within the range of from 2 to 5 and continuing said immersion for a period of from about minutes to about 3 hours depending on the thickness of the palladium deposit required.

12. A method according to claim 11 in which the Weak acid salt is selected from the group of salts consisting of sodium citrate, sodium acetate, sodium oxalate, sodium tartrate, sodium phosphate, sodium borate, sodium metaborate and the corresponding potassium salts.

13. A method for the immersion or electroless deposition of palladium on a metal base, said method comprising immersing the base in'a plating bath maintained at a temperature of about 2-0-70 C., said bath consisting essentially of an aqueous solution of a complex nitritopalladite compound having a general formula selected from the group wherein A represents a cation selected from the group consisting of alkali metal and hydrogen cations;

R represents a m-onovalent acid radicle;

R" represents a divalent acid radicle, and

a is a number selected from the group of numbers 1, 2 and 3,

and said solution containing about 1 to grams/litre of palladium and having been buffered to a pH within the range of from 2 to 5 by the addition to said solution of a buffering agent comprising a weak acid and an alkali metal hydroxide as required and continuing said immersion for a period of from about 15 minutes to 3 hours depending on the thickness of the palladium deposit required.

14. A method according to claim 11 in which the complex nitrito-palladite compound is selected from the group consisting of potassium-dichloro-dinitrito-palladite, K Pd(NO Cl and sodium-dichloro-dinitritopalladite Na Pd 2C12.

15. A method according to claim 11 in which the complex nitrito-palladite compound is selected from the group consisting of potassium-sulphato-dinitrito-palladite, K Pd(NO SO and sodium-sulphato-dinitrito-palladite 16. A method according to claim 11 in which the complex nitrito-pall adite compound consists of dinitrito-sulphato-palladous acid, H Pd(NO SO 17. A plating bath for the immersion or electroless deposition of palladium on a metal base, said bath consisting essentially of an aqueous solution of potassium- 6 sulphato-dinitrito-palladite K Pd(NO SO and containing 0.8 gram/litre of palladium and 2.46 grams per liter of sodium acetate, as a buffering agent, said bath having a pH of 3.28.

18. A plating bath for the immersion or electroless deposition of palladium on a metal base, said bath consisting essentially of an aqueous solution of potassiumsulphato-dinitrito-palladite K Pd(NO SO and containing 7.7 grams/litre of palladium and 0.82 gram per liter of sodium acetate, as a buffering agent, said bath having a pH of 3.8.

19. A plating bath for the immersion or electroless deposition of palladium on a metal base, said bath consisting essentially of an aqueous solution of potassiumsulphato-dinitrito-palladite K Pd(NO SO and containing 1 gram/litre of palladium and 0.4 gram per liter of sodium acetate, as a buffering agent, said bath having a pH of 4.18.

20. A plating bath for the immersion or electroless deposition of palladium on a metal base, said bath consisting essentially of an aqueous solution of potassiumsulphato-dinitrito-palladite K Pd(NO SO and containing 10 grams/ litre of palladium and 4.2 grams per liter of Rochelle salt as a buffering agent, said bath having a pH of 3.77.

21. A plating bath for the immersion or electroless deposition of palladium on a metal base, said bath consisting essentially of an aqueous solution of potassiumsulphato-dinitrito-palladite K Pd(NO SO and containing 25 grams/litre of palladium and 8.46 grams per lite-r of Rochelle salt as a buffering agent, said bath having a pH of 3.36.

22. A plating bath for the immersion or electroless deposition of palladium on a metal base, said bath consisting essentially of an aqueous solution of potassiumsulphato-dinitrito-palladite K Pd(NO SO and contain ing 10 grams/litre of palladium and 10 grams/litre of citric acid and 4.6 grams/litre of potassium hydroxide as buffering agents, said bath having a pH of 3.25.

23. A plating bath for the immersion or electroless deposition of palladium on a metal 'base, said bath consisting essentially of an aqueous solution of potassium- References Cited by the Examiner UNITED STATES PATENTS 2,720,494 10/1955 Suter 204-47 3,206,382 9/1965 Wilson et al. 23-50 OTHER REFERENCES Gmel-in: Handbuch Der Anorganischen Chemie, 1942, Ver Lag Chemie, Berlin, Germany, page 315, 8th edition. I v ALEXANDER H. BRODMERKEL, Primary Examiner.

J. E. CARSON, L. HAYES, Assistant Examiners.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No, 3,285,754 November 15, 1966 Norman Maynard Hopkin It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

line 58, for that portion of the formula reading Column 3,

A [Pd)NO read A [Pd(NO column 5, line 3, before "and" insert a closing bracket; column 6, line 2, for "008" read 9.8

Signed and sealed this 19th day of September 1967.

(SEAL) Attest:

ERNEST W. SW'IDER Attesting Officer EDWARD J. BRENNER Commissioner of Patents 

1. A PLATING BATH FOR THE IMMERSION OR ELECTROLESS DEPOSITION OF PALLADIUM ON A METAL BASE, SAID BATH CONSISTING ESSENTIALLY OF AN AQUEOUS SOLUTION OF A COMPLEX NITRITO-PALLADITE COMPOUND HAVING A GENERAL FORMULA SELECTED FROM THE GROUP: (A)2(PD(NO2)A(R'')(4-A)) AND (A)2(PD(NO2)A(R")((4-A)/2)) WHEREIN: A2 REPRESENTS A CATION SELECTED FROM THE GROUP CONSISTING OF ALKALI METAL AND HYDROGEN CATIONS; R'' REPRESENTS A MONOVALENT ACID RADICLE; R" REPRESENTS A DIVALENT ACID RADICALE, AND A IS A NUMBER SELECTED FROM THE GROUP OF NUMBERS 1, 2 AND 3, SAID SOLUTION CONTAINING FROM 1 TO 30 GRAMS PER LITER OF PALLADIUM, AND SAID SOLUTION ALSO CONTAINING A BUFFERING AGENT COMPRISING AN ALKALI METAL SALT OF A WEAK ACID IN SUFFICIENT AMOUNT TO GIVE SAID SOLUTION A PH WITHIN THE RANGE OF FROM 2 TO
 5. 